Ziegler-Natta (ZN) type polyolefin catalysts are well known in the field of polymers, generally, they comprise (a) at least a catalyst component formed from a transition metal compound of Group 4 to 6 of the Periodic Table (IUPAC, Nomenclature of Inorganic-Chemistry, 1989), a metal compound of Group 1 to 3 of the Periodic Table (IUPAC), and, optionally, a compound of group 13 of the Periodic Table (IUPAC) and/or an internal donor compound. ZN catalyst may also comprise (b) further catalyst component(s), such as a cocatalyst and/or an external donor.
Various methods for preparing ZN catalysts are known in the state of art. In one known method, a supported ZN catalyst system is prepared by impregnating the catalyst components on a particulate support material. In WO-A-01 55 230, the catalyst component(s) are supported on a porous, inorganic or organic particulate carrier material, such as silica.
In a further well known method the carrier material is based on one of the catalyst components, e.g. on a magnesium compound, such as MgCl2. This type of carrier material can also be formed in various ways. EP-A-713 886 of Japan Olefins describes the formation of MgCl2 adduct with an alcohol which is then emulsified and finally the resultant mixture is quenched to cause the solidification of the droplets.
Alternatively, EP-A-856 013 of BP discloses the formation of a solid Mg-based carrier, wherein the Mg-component containing phase is dispersed to a continuous phase and the dispersed Mg-phase is solidified by adding the two-phase mixture to a liquid hydrocarbon.
The formed solid carrier particles are normally treated with a transition metal compound and optionally with other compounds for forming the active catalyst.
Accordingly, in case of external carriers, some examples of which are disclosed above, the morphology of the carrier is one of the defining factors for the morphology of the final catalyst.
One disadvantage encountered with the supported catalyst systems is that a possible surface treatment (impregnation step) of the support with one or more catalytically active compounds may lead to non-uniform distribution of the active component(s) and in turn to an inhomogeneous polymer material.
WO-A-00 08073 and WO-A-00 08074 describe further methods for producing a solid ZN-catalyst, wherein a solution of a Mg-based compound and one or more further catalyst compounds are formed and the reaction product thereof is precipitated out of the solution by heating the system. Furthermore, EP-A-926 165 discloses another precipitating method, wherein a mixture of MgCl2 and Mg-alkoxide is precipitated together with a Ti-compound to give a ZN catalyst.
EP-A-83 074 and EP-A-83 073 of Montedison disclose methods for producing a ZN catalyst or a precursor thereof, wherein an emulsion or dispersion of Mg and/or Ti compound is formed in an inert liquid medium or inert gas phase and said system is reacted with an Al-alkyl compound to precipitate a solid catalyst. According to examples said emulsion is then added to a larger volume of Al-compound in hexane and prepolymerised to cause the precipitation.
In general, a drawback of such precipitation methods is the difficulty to control the precipitation step and thus the morphology of the precipitating catalyst particles.
Furthermore, the precipitation of the catalyst component(s) may often proceed via a “tar-like” intermediate stage. Said undesired sticky precipitate agglomerates easily and sticks to the walls of the reactor. The morphology of the catalyst would then of course be lost.
It is also known for some ZN-catalysts known in the art that the catalysts have their activity maximum at a relatively low temperature leading to a decreased or even drastically decreased catalyst activity, if polymerization is desired to carry out at higher temperature.
Accordingly, although much development work has been done in the field of Ziegler-Natta catalysts, there remains a need for alternative or improved methods of producing ZN catalysts with desirable properties.
It is of particular interest to obtain a catalyst in particulate form which results in good and desired polymer properties also when used in polymerization processes, where higher temperature are used. I.e. the catalyst activity maximum is shifted to a higher temperature.